Propulsion system and method for a cable ferry

ABSTRACT

The invention provides a flywheel-based propulsion system for cable ferries and method of operating such a propulsion system. A flywheel is energized from an on-shore source of electrical power during unloading and loading of the ferry and the flywheel is then used to generate electrical power for an electrical motor to power the ferry during the crossings.

REFERENCE TO RELATED APPLICATIONS

The present application claims the benefits, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 60/955,910 filed Aug. 15, 2007 which is incorporated herein by this reference.

TECHNICAL FIELD

The invention relates to the field of cable ferries, and in particular self-propelled cable ferries.

BACKGROUND

Cable ferries, also referred to as chain ferries, have long been used to transport vehicles and people for relatively short distances across bodies of water. Such ferries are guided by a cable, typically a wire cable, which is fixed to the shore at both ends and also serves as the means of propulsion. They are common for crossing rivers where a strong current would make navigation difficult for an unguided ferry, but can also be used for crossing wider bodies of water where marine traffic is not heavy. The cables have sufficient slack so that they sink thirty feet or more below the surface at locations away from the ferry to allow marine traffic to pass over the submerged cables.

Where the cable ferry crosses a fast-moving river, the ferry can use the river current alone to propel the ferry. In other cases the cable ferry uses an onboard internal combustion engine to drive a sheave which pulls the ferry along the cable, or the passengers can pull themselves along the cable by hand. Such internal combustion engines, whether gasoline or diesel, have the disadvantage of noise and emissions.

The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

The present inventor has discovered that the use of one or more flywheels for storing energy to provide electrical power to propel a cable ferry has surprising advantages. The duty cycle and logistics of a cable ferry are well-suited for energizing a flywheel from an on-shore source of electrical power during unloading and loading of the ferry and using the flywheel to generate electrical power for an electrical motor to power the ferry during the crossing.

The invention therefore provides a propulsion system for a cable ferry comprising: i) a variable frequency electric drive and motor configured to propel said ferry along a cable; ii) a flywheel; iii) an electric charging motor connected to the flywheel for selectively driving the flywheel; iv) an electric generator connected to the flywheel for selectively being driven by the flywheel to generate electrical power; v) means for connecting the charging motor to an external source of electric power; vi) an electrical bus for connecting the generator to the electric drive; vii) switch means for selectively connecting and disconnecting the output of the generator to the drive motor and the external power source to the charging motor; and viii) control means.

The invention further provides a method of operating a cable ferry, wherein the cable ferry is propelled by an electric motor and comprises a flywheel-based energy storage unit, the method comprising: i) charging the flywheel-based energy storage unit by connection to a source of electric power; ii) powering the propulsion of the ferry during a ferry crossing with electric power from the charged flywheel-based energy storage unit; iii) re-charging the flywheel-based energy storage unit by connection to a source of electric power while the car ferry is docked after the crossing; and iv) repeating steps ii) and iii).

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 is an elevation view of a cable ferry according to the invention showing the guide cable and diesel engines and bull wheels as a partial cross-section.

FIG. 2 is an end view of the cable ferry shown in FIG. 1.

FIG. 3 is a top plan view of the cable ferry shown in FIG. 1.

FIG. 4 is a schematic diagram illustrating the invention.

FIG. 5 is a detail plan view illustrating an arrangement of the power storage units and cable within the hull of the ferry.

DESCRIPTION

Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

With reference to FIG. 1, cable ferry 10 has a hull 12, deck 14 to carry cars or passengers, and wheelhouse 16. Guide cable 18 and drive cable 22 extend through hull 12. Cables 18 and 22 are secured at either end at shore-based ferry terminals. Drive cable 22 passes through the hull 12 and over one or more bull wheels 24 (FIGS. 4, 5), which are large diameter pulley wheels. Drive cable 22 may also pass over idler wheels. In the embodiment shown in FIG. 5, which uses a pair of drive bull wheels 24, when the ferry is operating in one direction one of the bull wheels functions as the drive wheel and the other as idler wheel 23, and when the ferry operates in the reverse direction the first bull wheel functions as the idler wheel 23 and the second bull wheel 24 is the drive wheel. A combination of wheels increases the angle of wrap of the cable 22 on the bull wheel 24. The friction of the cable on the bull wheel 24 provides the pulling tension on the cable 22, and an increased angle of wrap, increases the tension that can be applied to the cable 22.

The standard conventional power supply for a cable ferry is a diesel engine 26. The bull wheel 24 rotates slowly so it is driven through a right angle gearbox 28. The engine drives the gearbox 28 via a clutch/torque converter unit 30. There is an air brake 25 on the bull wheel drive shaft to hold the ferry 10 stationary at the dock. The right angle gearbox 28 prevents the bull wheel 24 from driving the diesel engine 26 and the clutch/torque converter 30 allows the diesel engine 26 to be started, idle in dock and brought up to sufficient RPM to produce adequate torque to move the ferry 10. The converter provides a smooth take off power on the cable when the ferry starts off.

The electrical propulsion system described as follows may be operated alongside the diesel engine 26 The system may be operated however without the diesel engine 26.

The basic principle of the present electric propulsion system is to store energy on board the ferry while the ferry is in the dock, and to use that power to drive an electric motor while the ferry is in transit. Cable ferry operation is cyclical, repetitive and consistent in its timing. Each crossing consists of a short acceleration time, followed by a period of running at a fixed speed and then slowing down into a dock. The crossing is typically completed in a matter of minutes. At the dock, time is required to unload and then load the vessel. The electric propulsion system according to the invention is designed to take advantage of this operating mode. The energy required for the crossing is known and consistent allowing an energy storage system to have a definitive size. The time spent in the dock allows sufficient time for the energy system to be recharged. The major components of the system are shown in FIG. 4, wherein solid lines represent power cable or electrical busbars, and dotted lines represent mechanical shafts.

There is a shore-based power supply 40 at each ferry terminal. This may be the community power grid or a dedicated power source such as a generator. AC electrical power may be provided to a transformer from the existing power grid 32, such as from hydroelectric power. At each shore-based power supply 40, the transformer feeds the AC power to an AC to DC converter 34. The converter 34 is sized to deliver power at a rate adequate to re-energize the ferry in the short time it is at the ferry terminal. A suitable design requires a power supply from converter 34 of approximately 1000 A @ 500 V DC for 5 to 10 minutes. The converter 34 is controlled by the control system 38 on the bridge of the ferry. There is an electrical connector 36 at each end of the ferry to connect to power supply 40 when the ferry docks. Once connected, a signal is sent to the control system 38 that re-energizing can commence.

To control the power flow from or to the storage units 44, a changeover switch 42 reroutes the electrical circuit from the shore connector 36 to supply power to the drive system. The storage units 44 are preferably designed to operate on DC voltage. To provide adequate energy storage, multiple storage units may be required, or a single large unit may be used. A DC bus 46 is the common connection point for all the drives. The control system 38 controls the voltage of the bus 46 and enables the power flow to or from each storage unit 44 to be controlled.

There will be one or more, and preferably several energy storage units 44. Each storage unit 44 consists of a high speed motor generator unit consisting of an AC motor/generator 48, drive shaft 50, DC/AC converter 52 and a flywheel 54 contained within a vacuum. Energy is stored by rotating the heavy flywheel 54 at very high speeds. The AC/DC converter 52 is a variable speed AC drive operating in four quadrants so that the AC motor/generator 48 functions as either a motor or generator. The control system 38 for each unit 44 will run the motor/generator as a motor putting energy into the flywheel by gradually increasing the speed of the flywheel. The AC/DC converter 52 converts the DC power received from the power supply 40 into the variable AC power to drive the motor 48. When energy is being removed from the flywheel 54, the control system 38 will reverse the operation of the motor 48 into a generator and control the voltage on the DC bus 46 as the flywheel 54 slows as it delivers energy to the electric drive motor 58.

In one embodiment of the invention, each unit 44 is a 50 kW flywheel system, with 1 kwH energy storage as manufactured by Flywheel Energy Systems Inc. Such systems incorporate as one unit the flywheel, motor/generator and a DC/AC converter, which is an IGBT based, variable frequency, bi-directional inverter/rectifier employing a 16 kHz switching frequency and sensorless rotor position feedback. The DC input/output of the units is connected to the DC bus 46. Other arrangements of flywheels and drive systems will be suitable for differing energy storage and output requirements, depending on the size of the ferry load and length of the crossing.

When the ferry is operating, the DC power supplied by storage units 44 is fed to a variable frequency drive 56. The power is converted back to AC by the variable frequency drive 56 to allow the use of a standard AC motor 58 to drive the bull wheel 24. The drive 56 uses advanced vector control to enable the motor 58 to produce maximum torque at zero speed and continue supplying the torque as the ferry ramps up to operating speed. Existing variable frequency drives which operate at high levels of efficiency can be used. Thus the flywheel 54 provides stored energy at high voltages and frequency to the AC/DC converter 52. The AC/DC converter 52 takes the power at high voltage and frequency from the flywheel 54 and provides power at a stable DC voltage on the DC bus 46 that can be used by the variable frequency 56 drive to produce power at the required voltage and frequency for the drive motor 58. The variable frequency drive 56 provides the drive motor 58 with low voltage, low frequency power to start the ferry moving. To speed up, the variable frequency drive 56 increases the voltage and frequency. As the flywheel 54 gives up energy, it slows, lowering its voltage and frequency.

A standard heavy duty AC motor 58 is used to propel the ferry. Designed to be powered by the variable frequency drive 56, the motor 58 supplies smooth power to accelerate the ferry up to its chosen operating speed. The motor 58 is chosen to be robust and suitable for a marine environment. The AC motor 58 may be directly connected to the gearbox 28 or directly to the opposite side of the bull wheel 24 from the diesel motor 26 as shown in FIG. 5. To enable the unit to be driven by the diesel motor 26 as a back-up, a disconnect clutch 60 will allow the motor 58 to be mechanically uncoupled. Since the unit will be isolated when not under load, the disconnect 60 can be simple.

The mechanical gearbox 28 is required to reduce the rotational speed of the motor to the speed required by the bull wheel 24. The standard gearbox for diesel power may be modified to have a double input shaft to allow the electric motor to also be connected. Since the electric motor 58 does not require a right angle gearbox 28, the system without diesel motor 26 may use a simpler gearbox or be directly connected to the bull wheel shaft 62. It may be useful however to maintain a conventional diesel engine 26 as a back up to the electric drive system should the energy storage system run down or if the electric propulsion system has a breakdown. As noted above, in this arrangement the bull wheel 24 is directly coupled to the gearbox 28. There is a parking brake 63 on the drive shaft 62, to hold the ferry stationary in dock.

FIG. 5 illustrates an arrangement wherein two bull wheels 24 are driven by two sets of storage units and drive motors 44, 56, 58 as well as two back-up diesel units 26. One or both back-up diesel units 26 may also be used to generate power to recharge the flywheels 54 by the addition of a generator with controls, and mechanical shafts and clutches. This would allow the flywheel to be recharged during crossing. Also a separate on-board generator may be provided to provide electrical power for other functions requiring electric power during crossing as well as recharging the flywheels if necessary. Alternatively, one of the diesel engines 26 shown in FIG. 5 could be replaced by a diesel Gen-set to drive the electric motor 58 as a back-up for short periods an/or recharge the flywheel.

Control of the electric propulsion system is from a control system console 38 on the bridge or wheelhouse 16. A distributed control system is preferably used, with individual controls on the storage units 44, variable frequency drive 56, power source 40 and changeover switch 42. These connect to the supervisory control system 38 on the bridge wheelhouse 16 that co-ordinates all the operations and operate the ferry as requested by the crew, plus giving all the status information required.

There are a number of advantages to the present invention over the conventional diesel system. The conventional diesel engine driven system has some undesirable characteristics, compared to an electric motor. It has to start up, idle to bring it up to operating temperature and continues to idle when the ferry is in dock. When the diesel engine is engaged as a drive, it has to be running with sufficient revolutions to provide adequate torque for pulling the ferry. This requires the use of a clutch/torque converter to allow the motor to run disconnected from the gearbox. This converter will also cushion the impact of engaging the drive so that the ferry will have smooth take off. A diesel engine cannot be driven by its load. When the ferry is slowing down when approaching the dock there is a potential for its momentum to attempt to drive its propulsion system. The right angle gearbox is required to prevent this happening.

There are further advantages of the present system over conventional diesel power, as follows.

-   -   i) The ferry will not produce any fumes during operation or in         dock. Diesel powered cable ferries idle their engines while         docked and waiting to load or to depart on a schedule whereas a         flywheel powered cable ferry only consumes such power caused by         the minimal frictional losses occurring in the flywheel units.     -   ii) The electrical storage system is primarily metal         construction with water as the principle fluid, so in the event         of the vessel sinking or becoming flooded, very little pollution         would result. While the ferry may still carry diesel fuel, since         it is only for back up, then quantities can be limited.     -   iii) Operation is quiet.     -   iv) There is a reduction in operating costs. Clean hydro power         can be used compared to diesel engine operation which has         limited efficiency.     -   v) There is a reduction in manpower and maintenance for         electrical equipment compared to diesel engines.     -   vi) Ferry internal equipment can be redesigned to simplify         mechanical equipment (Diesel back up can be a diesel electric         generator set).     -   vii) Direct electrical drive is more efficient than a torque         converter.     -   viii) Vector controlled variable frequency drive will provide         smooth ferry operation with gentle start up and docking.     -   ix) The electrical drive system is easier to convert to a         totally automated ferry operation.

Other forms of energy storage than flywheels that might be used, including chemical (batteries) and fuel cells, do not have the capacity for the repeated charge/discharge cycling required by the ferry run operations cycle. The energy efficiency is much lower than the fly wheel storage (efficiency is the ratio of energy to charge the storage to the energy extracted during discharge). Chemical storage has environmentally sensitive fluids that could enter the environment should the ferry become flooded or sink. The flywheel storage system as described above is designed for efficient frequent deep cycling of power reliably for extended periods of time. Very high charge/discharge rates can be used, providing flexibility of operation without any adverse impact on the storage system.

While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the invention be interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. 

1. A propulsion system for a cable ferry comprising: i) a variable frequency electric drive and motor configured to propel said ferry along a cable; ii) a flywheel; iii) an electric charging motor connected to said flywheel for selectively driving said flywheel; iv) an electric generator connected to said flywheel for selectively being driven by said flywheel to generate electrical power; v) means for connecting said charging motor to an external source of electric power; vi) an electrical bus for connecting said generator to said electric drive; vii) switch means for selectively connecting and disconnecting the output of said generator to said drive motor and the external power source to said charging motor; and viii) control means.
 2. The propulsion system of claim 1 wherein said charging motor and generator are combined in a motor/generator which selectively functions as either a motor or generator.
 3. The propulsion system of claim 2 wherein said motor/generator is AC.
 4. The propulsion system of claim 3 further comprising a DC/AC converter electrically connected to said motor/generator.
 5. The propulsion system of claim 2 comprising a plurality of flywheel-based energy storage units, each comprising said flywheel, motor/generator and DC/AC converter.
 6. The propulsion system of claim 1 wherein said electrical bus is DC.
 7. The propulsion system of claim 1 wherein said electric drive motor is an AC motor.
 8. The propulsion system of claim 1 further comprising a variable frequency drive electrically connected to said drive motor to convert DC power from said generator to AC power.
 9. The propulsion system of claim 1 further comprising an AC to DC converter to deliver power at a rate adequate to re-energize the ferry in the short time it is at the ferry terminal.
 10. The propulsion system of claim 1 further comprising a back-up diesel motor and generator configured to deliver electric power to said charging motor.
 11. A method of operating a cable ferry, wherein said cable ferry is propelled by an electric motor and comprises a flywheel-based energy storage unit, said method comprising: i) charging said flywheel-based energy storage unit by connection to a source of electric power; ii) powering the propulsion of the ferry during a ferry crossing with electric power from said charged flywheel-based energy storage unit; iii) re-charging said flywheel-based energy storage unit by connection to a source of electric power while said car ferry is docked after said crossing; iv) repeating steps ii) and iii).
 12. The method of claim 10 further comprising the steps of providing a diesel motor-powered generator and initiating said generator to re-charge said flywheel-based energy storage unit if required during one of said crossings.
 13. The method of claim 11 wherein said flywheel-based energy storage unit comprises a flywheel, an AC motor/generator and DC/AC converter. 